Honeycomb structures are in use in, for example, a filter for capturing fine particles present in an exhaust gas emitted from an internal combustion engine, a boiler or the like, particularly diesel fine particles.
As shown in FIG. 5(a) and FIG. 5(b), honeycomb structures used for such a purpose have, in general, a large number of through-holes 3 divided by partition walls 2 and extending in an X axis and have such a constitution that each adjacent through-holes 3 are plugged at opposite ends of the structure each other (plugged alternately) so that each end face looks checkerboard pattern. In a honeycomb structure having such a structure, a fluid to be treated enters those through-holes 3 not plugged at the inlet side end face 42, that is, those through-holes 3 plugged at the outlet side end face 44, passes through porous partition walls 2, and is discharged from adjacent through-holes 3, that is, those through-holes plugged at the inlet side end face 42 but not plugged at the outlet side end face 44. In this case, the partition walls 2 become filters and, for example, soot emitted from a diesel engine is captured by the partition walls and deposited thereon. In a honeycomb structure used in such a way, the sharp temperature change of exhaust gas and the local heating of the structure make non-uniform the temperature distribution inside the structure and there have been problems such as crack generation in honeycomb structure and the like. When the honeycomb structure is used particularly as a filter for capturing a particulate substance in an exhaust gas emitted from a diesel engine (this filter is hereinafter referred to as DPF), it is necessary to burn the carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are inevitably generated locally in the filter; as a result, there were problems of a reduction in filter regeneration efficiency due to non-uniformity of regeneration temperature and easy generation of cracks due to large thermal stress. Further, non-uniform temperature distribution during filter regeneration made it difficult for the whole portion of filter to be at an optimum temperature and also made it difficult to achieve a high regeneration efficiency.
Hence, there were proposed processes for producing a honeycomb structure by bonding a plurality of individual segments using an adhesive. In, for example, U.S. Pat. No. 4,335,783 is disclosed a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. Also in JP-B-61-51240 is proposed a heat-shock resistant rotary regenerative heat exchanger is formed by extrusion molding a matrix segment of honeycomb structure made of a ceramic material; firing them, making smooth, by processing, the outer peripheral portion of the fired segment; coating the parts to be bonded of the resulting segments with a ceramic adhesive which turns to have substantially the same mineral composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800° C., after firing; and firing the coated segments. Also in a SAE article 860008 of 1986 is disclosed a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with cordierite cement. Further in JP-A-8-28246 is disclosed a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles. Also, it was tried to prevent the local heating of honeycomb structure and its breakage caused by thermal stress, by producing a honeycomb structure using, for example, a silicon carbide-based material of high thermal conductivity and high heat resistance.
By using honeycomb segments and/or using a highly heat-resistant material such as silicon carbide-based material, breakage caused by thermal stress can be prevented to a certain extent; however, the temperature difference between outer peripheral portion and center of honeycomb structure cannot be eliminated and the above approach has been insufficient in uniform regeneration and resultant durability improvement. Further, local heating appeared during regeneration, in some cases.
In JP-A-2001-162119 is disclosed a filter which is a ceramic filter assembly having a sealing layer (a bonding layer) of 0.3 to 5 mm in thickness and 0.1 to 10 W/mk in thermal conductivity, thereby can have a uniform temperature as a whole, and gives little local unburned matter. By keeping the thickness of bonding layer and the thermal conductivity each in a given range, local unburned matter can be made small and an increase in soot regeneration efficiency is made possible; however, the above filter was insufficient in prevention of the generation of temperature gradient and thermal stress when there was heat generation to high temperatures locally, and insufficient in an increase in maximum soot amount enabling soot regeneration.